1. Field of the Invention
This invention relates to a transmitter for transmitting transmit signals. More specifically, this invention relates to a transmitter that effectively suppresses the peak level of transmit signals.
2. Description of the Related Art
In a wireless communication system transmitter that utilizes CDMA (Code Division Multiple Access), for example, the peak level of the transmit signals is suppressed.
An example that can be cited explains that in a carrier synthesizing transmission circuit equipped with a limiter circuit, defining the ratio of the instantaneous power of the signal obtained by multiplexing all carriers to the average power thereof as the instantaneous peak factor, comparing the instantaneous peak factor with a reference value to determine a limit coefficient suitable for the required degree of clipping, and using the limit coefficient to apply required clipping to the instantaneous powers transmitted by the respective carriers before amplification by the power amplifier. (See, for example, Japanese Unexamined Patent Publication No. 2002-44054.)
In wireless communication and the like, however, since the frequency band used for communication is limited, a need arises to minimize the expansion of the frequency spectrum caused by the nonlinear distortion of the power and other amplifiers. This need in turn makes it necessary to operate the amplifiers in the linear region when, for example, amplifying code multiplex signals with the power and other amplifiers.
Moreover, consideration has been given to configuring conventional transmitters so as to limit the transmit signal level in the band-limited intermediate frequency (IF) region, for example. However, the conventional configuration has a problem in that it increases power leakage to outside the band more than it lowers the peak level.
It is also worth noting that reapplying band restriction to the output after carrying out peak reduction restores the original spectrum and also restores the original time waveform. This is a problem because the peak power limiting effect is also lost.